This is largely going to be related to how the individual sits for a pull. If anything, the lower back should be less likely to round because you begin your pull from a higher starting point. Which means you don't have to squat down as far. With a trapbar deadlift, your torso angle will/should be higher. This is what takes emphasis off the back and places it on the legs.

Not to say that trap bar deadlifts do not work your lower back.

From my experience every person who gets into a trap bar bows the back in the initial pull from the hips driving back rather than up. I threw the ones from our weight room in storage. Total junk in my opinion. If I am taking the time to teach someone to lift, I wouldn't waist my time on it.